A vehicle exhaust system conducts hot exhaust gases generated by an engine through various exhaust components to reduce emissions and control noise. The exhaust system includes an injection system that injects a reducing agent, such as a solution of urea and water for example, upstream of a selective catalytic reduction (SCR) catalyst. A mixer is positioned upstream of the SCR catalyst and mixes engine exhaust gases and products of urea transformation.
The injector typically sprays the urea into the exhaust stream. The spray makes contact with the mixer and surrounding walls which have been heated by the exhaust gases. As droplets of the urea contact these heated structures, the surfaces cool down which favors a local formation of urea deposits. These deposits can adversely affect flow circulation and operating efficiency.